This is typically caused by the shape of the tank, and the location of the float arm on the fuel pump assembly.
When the tank is full, the float arm stays maxed out at the topmost position (because the float is submerged in fuel) until you've burned enough fuel for the float arm to start dropping. Aaaand the fuel gauge won't start dropping until the fuel level is below the maximum float arm angle.
To those who don't know, the float arm is attached to a rheostat (variable resistor, a three-wire device -- power, ground, and signal) which alters electrical current based on the fuel level inside the tank. As the fuel level drops, the float arm drops, which causes a measurable voltage drop across the rheostat -- and that's what's driving the fuel level gauge.
It's horrible on cars with V-shaped fuel tanks. They report "full" forever because there's a lot of fuel volume at the top of the tank, but over time, the fuel level accelerates toward empty because the volume of the tank narrows toward the bottom. So you drive 20 miles and suddenly lose a quarter-tank of fuel on the gauge. Difficult to explain, but hopefully that makes sense.
My first bike didn't even have a fuel level gauge, so I'm thrilled with even somewhat inaccurate ones.